Journal of Earth Science & Climatic Change
Open Access

The global climate crisis is accelerating, and with it comes a growing need for frameworks that can guide policy and action. Among the most critical and scientifically grounded tools in climate science is the carbon budget. The carbon budget defines the maximum amount of carbon dioxide (CO₂) that can be emitted while still having a chance to limit global warming to a specific target, such as 1.5°C or 2°C above pre-industrial levels, as agreed under the Paris Agreement.

In simple terms, the carbon budget is a planetary limit. It is a ceiling on how much more we can emit before the climate crosses into potentially dangerous and irreversible territory. This article explores the concept of the carbon budget, its scientific foundations, how it guides policy decisions, the current status of global emissions, and the challenges and opportunities it presents for the future [1-3].

Understanding the Carbon Budget

The concept of a carbon budget is based on the linear relationship between cumulative carbon emissions and global temperature rise. The more carbon dioxide we emit, the more the planet warms. This correlation allows scientists to calculate a finite emissions "budget" that corresponds with a particular temperature threshold.

Key Components of the Carbon Budget

Total Budget – The total amount of CO₂ that can be emitted to stay within a target level of warming.

Emitted Budget – The amount of CO₂ that has already been released into the atmosphere.

Remaining Budget – The difference between the total and emitted budgets, representing how much more we can emit.

For example, to limit warming to 1.5°C, the Intergovernmental Panel on Climate Change (IPCC) estimates that the remaining global carbon budget from 2020 onward is approximately 400 gigatonnes of CO₂ (GtCO₂). At current global emission rates of around 40 GtCO₂ per year, this budget could be depleted within a decade [4].

The Carbon Budget and the Paris Agreement

The Paris Agreement, adopted in 2015, aims to limit global warming to “well below 2°C,” with efforts to limit it to 1.5°C. The carbon budget plays a vital role in operationalizing this target. It provides a quantifiable limit that guides countries on how much they can emit individually and collectively to stay within safe temperature thresholds.

However, the current pledges made under Nationally Determined Contributions (NDCs) are not sufficient. They put the world on a path toward 2.5–2.9°C of warming, far beyond what the remaining carbon budget allows for the 1.5°C goal. This "emissions gap" illustrates the disconnect between political ambition and climate science [5-7].

Scientific Basis of Carbon Budgets

• Carbon budgets are derived from climate models and observational data, combining several variables:
• Cumulative CO₂ emissions
• Climate sensitivity (how much warming occurs with a given CO₂ concentration)
• Non-CO₂ greenhouse gases like methane (CH₄) and nitrous oxide (N₂O)
• Aerosol effects, which can temporarily cool the atmosphere
• These models are refined using historical climate data and advanced simulations to project different emission scenarios.

Uncertainties in Carbon Budget Estimates

While the concept is straightforward, there are scientific uncertainties in the size of the budget due to:

Feedback loops (e.g., permafrost thaw, forest dieback)

Natural carbon sinks (oceans and forests)

Potential for large-scale tipping points

Despite these uncertainties, the carbon budget remains a powerful and practical tool for climate planning.

Global Emissions and the Shrinking Budget

The world has already used up more than 85% of the total carbon budget for 1.5°C. Annual CO₂ emissions remain high, driven primarily by fossil fuel combustion, industrial activity, and deforestation.

These countries, especially those with historically high emissions, have a disproportionate responsibility to reduce their carbon footprints and contribute to global efforts.

Equity and Fair Share in the Carbon Budget

One of the biggest debates around the carbon budget is how to allocate it fairly. Developing countries argue that industrialized nations have already used most of the budget through historical emissions. This raises questions about:

Climate equity: Should countries have equal per capita emissions rights?

Climate justice: Should high-income countries pay more for mitigation and adaptation in poorer nations?

Responsibility sharing: How can global cooperation reflect both capability and responsibility?

Many climate justice advocates call for a “fair share approach” that factors in historical emissions, capacity to act, and development needs.

Carbon Budget and National Climate Policies

Countries are increasingly using carbon budgets to set long-term climate targets. For example:

United Kingdom has legally binding carbon budgets set in five-year periods.

Germany enshrined its national carbon budget into law and established sectoral targets.

Sweden and Denmark have adopted carbon neutrality targets with pathways aligned to budget constraints.

However, many countries still lack clear strategies to stay within their fair share of the global budget. Policies are often inconsistent or based on short-term economic goals rather than science-based limits [8-10].

The Role of Negative Emissions Technologies (NETs)

Given the shrinking carbon budget, many climate strategies now include negative emissions technologies to remove CO₂ from the atmosphere, including:

Afforestation and reforestation

Bioenergy with carbon capture and storage (BECCS)

Direct air capture (DAC)

Soil carbon sequestration

While promising, these technologies are not yet scalable enough to offset significant emissions, and relying on them can create false security and delay necessary emissions cuts.

Carbon Budget at Local and Corporate Levels

The carbon budget framework is increasingly used by cities, companies, and institutions to set science-based climate targets:

Cities like Amsterdam and Melbourne have local carbon budgets aligned with the 1.5°C goal.

Corporations use Science-Based Targets initiative (SBTi) to align their emissions with the global budget.

Financial institutions are beginning to assess investment portfolios based on carbon exposure.

These efforts are critical because action at the subnational level can complement or even exceed national climate commitments.

Challenges to Carbon Budget Implementation

Despite its importance, implementing the carbon budget approach faces several hurdles:

Lack of Political Will – Many governments are unwilling to make the tough choices required to stay within the budget.

Economic Dependence on Fossil Fuels – Transitioning away from carbon-intensive industries threatens jobs and revenues.

Short-Term Thinking – Political cycles and profit motives often ignore long-term climate consequences.

Technological and Financial Gaps – Especially in the Global South, limited access to clean technologies hampers progress.

Opportunities and the Path Forward

While the carbon budget paints a stark picture, it also offers a roadmap for urgent and strategic action:

Rapid Decarbonisation: Shifting to renewable energy, electrifying transport, and decarbonizing industry.

Efficiency Improvements: Reducing energy use in buildings, transportation, and manufacturing.

Behavioural Changes: Encouraging low-carbon lifestyles, such as reduced meat consumption and less air travel.

Climate Finance: Mobilizing trillions in public and private capital to fund clean development, especially in vulnerable nations.

The earlier and faster the world cuts emissions, the more likely it is to remain within a safe carbon budget.

Conclusion

The carbon budget is not just a scientific concept—it’s a climate lifeline. It gives humanity a finite and measurable goal, clearly linking emissions today to the world we will live in tomorrow. With every passing year of inaction, the budget shrinks, and the challenge becomes steeper.

Staying within the carbon budget is not impossible, but it demands unprecedented global cooperation, political courage, and societal transformation. The urgency to act has never been clearer, and the roadmap has never been more precise. The real question is not whether we can stay within the budget—but whether we will.